If a file has custom tags created by another application you can use them in JRiver.
First you have to find out the name of this tag.

Select a single track, choose tag.
In the tag panel click on the file (detailed format information).

In this case there is a TXXX with epoque in it. This is the name of the custom tag.

In Tools > Options > Library & Folders add a field to the library with exactly the same name.

Now you can use the field in the interface.