You have two options to add new TUs to an existing TM.
If you have your translated bilingual files, you can just clean up a copy of this set of files selecting the Update TM option in the Clean Up window.
Another option is importing and exporting TMs. Suppose you want to "merge" two TMs, that is making one TM out of two, just export one of them using File > Export in Tr. Workbench. Then open the other TM and import the TM previously exported (in txt or tmx format) using File > Import.
Importing is a bit trickier than exporting, as you'll see there are many options like Merge, Overwrite, etc, so make sure you pick the right one.
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