Hi to all,
I wonder is it possible to use a firewall with mikrotik at the same ip range.

VDSL >>> "Firewall [(UTM),dhcp server,distrubute ip according to mac]192.168.1.a" >>> "Mikrotik[ hotspot ]192.168.1.b" >>> computers 192.168.1.xyz

please do not ask why, i need to use that system.

It kind of depends on the upstream address. For example: a=128-254 b=1 xyz=2-126 should work with a 7 bit netmask for the downstream "private" range. If you don't have control of the upstream address, it gets a lot harder.