Irrational time signatures in Sibelius?

Are these possible?

If, for example, I wanted a bar of 3/5, which would be 3 quintuplet crotchets, how could I do it? Mainly it is to avoid lots of ugly tempo changes.

Also, is it possible to split a bar into a rational and irrational part (divided by dotted barline), and write a time sig to describe this? For example, 4/4 + 2/6 would equate to a bar of four crotchets with two crotchet triplets at the end. All of these things have been done in notation before (Cowell, Ferneyhough, Thomas Ades), I just wonder if you can do them in Sibelius.

L.

Edit: Sorry, wrong forum.

Hit "T" on your keyboard to bring up the time signatuires menu. Then click "Other" and input whatever signature you want in the spaces provided.