Yes you can add special mosfet, or for that matter any mosfet specs, by just changing the values of the N-channel MOSFET (included in Electrical library on side panel) parameters. IRLML2502 is a N-channel MOSFET and you can easily model it using our N-Channel MOSFET element from electrical library. I encourage you to go through MOSFET section of the document “OptiSPICE_Element_Library” to understand the symbols of the model.
MCP6021 op-amp can also be build using traditional NPN OR PNP transistors available in the electrical library. You can even use your IRLML2502 once you have modelled it correctly and it gives you desired response (compare the response from the data sheet you referring). Once you create a final schematic for the MCP6021 op-amp, you can create a sub-circuit of the whole schematic with 5 ports (MCP6021 has 5 ports). You can go through the section “Creating Sub-Circuits” of the document “OptiSPICE_Tutorials-Advanced” in order to understand how to create a sub-circuit out of schematic.
I suggest you to check the internal schematic for the MCP6021 op-amp first and after you finish your schematic using mosfets, compare its performance from the data-sheet. Then create a sub-circuit out of it. Then you can save this sub-circuit into your own library by creating a new library (shown in section “Creating Sub-Circuits” of the document “OptiSPICE_Tutorials-Advanced”). You can also save this sub-circuit in any of the existing libraries as well.
Hope this helps!
Let me know if you have any further questions.