You cannot import the local files directly due to a file format change in T:ANE.
To import content from TS12:
1) Start up both T:ANE Content Manager and TS12 Content Manager.
2) Exclude Built-in and any DLC content.
3) Open up assets for edit in TS12 CM.
It's highly recommended you do this in small batches of 100 or 200 or so assets.
4) Open up Windows File Explorer.
5) Navigate to where your TS12 is located and go to the User Data/Editing folder.
6) Select all the open folders by pressing CTRL+A
7) Drag the folders into your T:ANE Content Manager.
The process can take some time depending upon how much memory you have and how fast your CPU is as this data needs to be converted.
Once this process is done, repeat as above with another batch of data - commit what was done previously and open up more.
The alternative is to open the content for editing in TS12 then use File-Import content from T:ANE's Content Manager File menu. Personally I find dragging content folders easier.
When all the importing is complete and in T:ANE Content Manager:
1) Revert to original any assets that are open for edit as these are built-ins now in T:ANE.
2) Keep in mind you will have errors so don't panic.
3) Download any missing dependencies.
4) Repeat the part I again and import more if necessary.
Using the process I outlined above, I imported close to 200,000 assets. In the end I reverted a bunch to original, and repaired about 7,500 assets. Today I show no errors in my content other than a few routes I've imported and are missing dependencies.
Speaking of error reporting in T:ANE, the faulty assets in T:ANE are listed in various ways which is confusing. I created a custom Faulty filter to display only Faulty assets and not missing dependencies and not faulty dependencies. This pared down my faulty list to far less than was initially displayed.
And yes, this process takes time. For nearly 200,000 assets not counting the repair stage, the importing took about 3 weeks. The repairing afterwards took around 6 weeks. You are lucky with the CRG and other content creators updating and repairing content, many of the assets that I repaired manually are now available as updates from the DLS. When repairing the assets, check that there isn't a new version available for download (right-click on the list and choose Asset Versions). If there is a new version, by all means download that to save the repairing process.
In the end when repairing the usual tools work fine. I highly recommend you download the following tools:
1) PEV's Tools - Images2TGA (latest version), PM2IM, and AssetX
2) Paint.net or some other graphics editor.
3) A text editor unless you prefer to use Notepad.
4) A Search and Replace utility for bulk text repairs. Search and Replace Master used to be available online, but that might no longer be around. There are others.
The purpose of a text replace utility is to allow multiple assets to be repaired that have the same config.txt error. This proves it's worth when you have 20 assets that have references to a_texture.tga when it should be a_texture.texture.
By far the most errors you will have will be related to typos in the config.txt files, incorrect image references, and missing thumbnails. What you will not be able to repair are LOD errors, or script errors. For these your only solution is to find the updated assets.
For splines, you'll find that there will be a lot of errors initially. These are related to the "default" mesh missing. The default mesh is actually the mesh file in the asset its self. By opening the asset for editing and renaming the mesh.im to default.im, this will repair that problem.
In the end once your repairs and imports are complete, I recommend clearing out the Obsolete content as that will only clutter up things.
And finally, start a new thread in the T:ANE support forum with any errors you do not know how to repair and one of us will assist, though I'm out of commission for a good while with eye issues which have kept me away from the forums most of the day.