In fact, I think it is possible. It is quite tricky but you have to superimpose another element on the UEL element. This other element will work with a UMAT.
To make the UMAT talk to the UEL and get the SDV, you ll need to write a COMMOM statement at the beginning of both subroutines that will include the values of the solution dep variables (SDV) for each element and each gauss point.
In the UMAT you put STRESS=0 and DDSDDE=0 and you just read what you get from the UEL as SDVs.
Hope the trick was understandable.
good luck if u need it