Hello Freerik,
You may consider storing the back stress in a state variable. That way, you are not constrained by what the subroutines may/may not offer.
Alternatively, see the ElemGetMat routine. It has variable storage for back stress: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v222/en/ans_prog/Hlp_P_UPFNEWEL.html
This subroutine is technically for a user-defined element, but it may work for pre-existing elements.
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