It is very straightforward to add a new MethodDef. You just need to create a new SPFPrimaryMethodDef and relate it to an interface via the 'performs' relationship.
Below is an example definition as used for IssueChange.
<PrimarySPFMethodDef>
<IObject UID="MD_IssueChange" Name="IssueChange" Description="Issues the change" />
<IPrimarySPFMethodDef />
<ISchemaObj />
<ISPFMethodDef MDArguments="" MDType="Sub" />
</PrimarySPFMethodDef>
<Rel>
<IObject UID="ITRNChange.MD_IssueChange" />
<IRel UID1="ITRNChange" UID2="MD_IssueChange" DefUID="Performs" IsRequired="False" OrderValue="0" />
<ISchemaObj />
</Rel>