They will TX on gmrs freq without modification. To use DMR repeaters you do need a DMR ID associated with your ham callsign.
However, for simplex dmr operation you can arbitrarily assign a unique radio number to each radio and realize the full dmr functions of the radio such as txt, remote monitor, encryption etc etc
I may be posting more on this soon, because you are not the 1st person to ask.
Here is a good video describing how you might do this...
DMR programming for concerned citizens