You can't convert it directly. If you have the source before it got exported then you can do somethings but realistically it's more work than starting over. For example one problem is the bogies in MSTS are part of the asset whilst in Trainz they are separate and added in the config.txt file. The MSTS stuff is quite elderly now by the way.
Very occasionally if you know exactly what you are doing it is possible to make a model for both. Normally you design it in such a way as you partially make it then clone and add the relevant bits for each.
Cheerio John