Create a Part from Circuit
See a movie of B2 Spice's create part feature in action 
B2 Spice v.5 features functionality for creating and storing custom parts. These parts can even be parametrized. Just follow the simple 3 step process to creating a new part.
1. Simply draw out the circuit for your new part.
2. Add in subcircuit port markers (found in the Common Parts -> Markers menu).
3. Choose the "Create Part from Circuit" feature found in the file menu and follow the prompts.