In Logisim, you can use VHDL to create new components called VHDL entities. Those components can then be used in the simulator, providing you have Questa Advanced Simulator installed and configured adequately.

See below for more information about the VHDL entities and how to use them in a simulation context.

VHDL Entity
Configuring Questa Advanced Simulator
VHDL simulator
Simulating test benchs

Next: VHDL Entity.