You mean adding a new solver except the ones that are there by default (dassl,euler,rk,etc)? You would need to modify the source code.

yes,and in the system guide I have seen that "
The library libsim.a is the runtime library for simulations, it contains solvers and a main function for the simulation. The following files are included:
simulation_runtime.* Includes the main function, solver wrappers,etc.
daux.f Auxiliary Fortran functions.
ddasrt.f DDASRT solver.
ddassl.f DASSL solver.
dlamch.f Determine machine parameters for solvers.
dlinpk.f Gaussian elimination routines, used by solvers.
lsame.f LAPACK axuiliary routine LSAME.
Non-linear solver:
hybrd1.f Non-linear solver with approximate jacobian.
hybrj.f Non-linear solver with analythical jacobian.- alternative for non-linear solver.
fdjac1.f Helper routines
enorm.f Helper routines.
dpmpar.f Helper routines
dogleg.f Helper routines"
and can i use these solver?for example, the non-linear ones?thank you!

The non-linear solvers are used to solve non-linear system. They are not used for numerical integration (which choosing a solver in OpenModelica implies).