Creating clock sources in SOPC Builder

I don't see why it wouldn't be possible, but it's true that the most common way to do that is either to have the pll outside of the SOPC project, or use the ready made pll component inside SOPC builder.

I just had a look at the component creator documentation and the avalon spec, and couldn't find any clock interface property or tcl command that would define the frequency of a clock lource so I don't know how they do that. Maybe instantiating a pll component in a SOPC builder project and have a look at the generated files could shed some light on that, but it doesn't seem to be documented.