Bachmann EZ track can easily be joined with flex track. You will have to cut off the clip and the alignment pins on one end of the section of EZ track you want to join to the flex track. You will also need some sort of road bed for the flex track. There are different codes of track. EZ track is code 100 so you will need to find some code 100 flex track.
You will also need to ‘shimmy’ the roadbed for the flex-track. The molded roadbed of the HO EZ track is about 1/8" higher than most commercial foam or cork roadbed. I have a section on my own MR that is laid with EZ 36" radius curves and in order to get my flex-track to attain the same roadbed height, I had to gradually raise the flex-track roadbed with ‘filler’ pieces of sheet styrene.
Well except that you need to cut it to length, which isn’t always easy on curves, and you need to install roadbed underneath the track and attach it to the roadbed…and attach the roadbed to the subroadbed base. Then of course if you change your mind and need to move the track over a few inches you need to rip the whole shebang out and start over with the cork roadbed.
Unless you have a situation where you absolutely can’t do it any other way than using flex track, you may find it easier to stay with the Bachmann track.