Setup a filter in Content Manager to hide built-in and packaged assets and then sort the assets by author to keep them organized.
High-light and open up a single author's assets.
Using Windows Explorer, you can then drag those assets into TRS22 Content Manager.
Once the content is imported, revert those open assets in TRS19 Content Manager to original and then repeat the process with another batch.
The alternative to dragging and dropping is to use a batch file to automate the process using Trainz Util. I did this to import my content into TRS19.
PS C:\Program Files\N3V Games\Trainz Railroad Simulator 2019> ./trainzutil installfrompath F:\TANE_DATA\editing
Substitute the bold items from your TRS22 program and data location.
TIP:
I more than highly recommend opening up only a few hundred assets at a time. The import process really pushes the CPU to 100% utilization and really eats up the RAM. You may also want to increase the size of your page file as well. I have found that really helps as well for this kind of work. If you are using a regular hard drive, you should stop and periodically defragment the drive. I found that really helps with the performance.