If you are using it as a latent variable, you can do the deterministic transformation on the Beta:
with pm.Model():
p = pm.Beta('p', 1., 1.,)
p_ = a + p_*(b-a) # p_ is now in the range of (a, b)
You can add Jacobian correction to a potential to make sure it is proper.
If you are using the Beta as observed, then the easiest way is scale the observed to (0, 1)