IBIS model of capacitors
I cannot answer to the Altium part of your question.

Hyperlynx permits the use of a simple model (built in perfect capacitor), an IBIS model file (where you can define the parameters) or by using a S-parameter model.

You can assign an IBIS model, S parameter or perfect capacitor from this dialog:



S parameter models are available from TDK and AVX and perhaps others; these will probably give you the best results for signal integrity. Hyerlynx supports the Touchstone format.