It is not possible to create a TM from existing jobs without exporting and re-importing the MXLIFF files into a new TM.
Currently, the only way to preserve TUs metadata is to export the MXLIFF files, create a new TM and import the MXLIFF files into the new TM.
Add an option to create a new TM with all confirmed segments from selected jobs (ideally from the Jobs table).
Otherwise, manually create a new TM from the Project page (as usual), and add an option to populate it with confirmed segments from selected jobs to the Jobs table.
In case a Project was created without TM, it would be a faster way to get an updated TM while the translation process has already started/finished.