What do you mean by “extendedDomain” function?
In any case, OpenLB can handle an arbitrary number of STL files as its “STL support” is implemented as the STLreader
indicator. What you do with those indicators and which other library functions you use them in is left up to you.
The “solver part” of OpenLB by design doesn’t know anything about STLs and as such doesn’t restrict how you use STLs in the simulation setup phase.