There are multiple ways of doing this. The most portable and recommended way would be to use mov reg, imm:
using namespace asmjit;
using namespace asmjit::host;
// You have to initialize these...
Compiler c;
GpVar var(c, kVarTypeIntPtr);
void* p = NULL;
c.mov(var, imm_ptr(p));
Or lea reg, mem having an absolute address [mem] form. This solution works as expected only in 32-bit mode; the absolute address size is always truncated to 32 bits:
c.lea(var, ptr_abs(p));