Can the access policies defined using different access structures (Monotone Span Program and Access Trees) be used together in CP-ABE scheme?
Yes. As Attribute-based signcryption with hybrid access policy (Yu and Cao, 2015) shows, it is possible to combine an Attribute-based Encryption scheme with an Attribute-based Signature scheme of different access structures and different flavors. This is commonly referred to as Attribute-based Signcryption.
What changes are necessary for the two access policy structures to work together?
That would be a longer answer along with the necessary security game.
For example, can I translate an MSP access policy to an access tree?
Yes, because a monotone access tree has the same expressiveness as a Monotone Span Program. I don't know whether there is an algorithm for this. It is usually done the other way around. For example, access tree to LSSS is described in Efficient Generation of Linear Secret Sharing Scheme Matrices from Threshold Access Trees (Liu, Cao, Wong, 2010).
Note that a Monotone Span Program (MSP) is equivalent to a Linear Secret Sharing Scheme (LSSS). Again, I'm not sure if there is an algorithm of doing the translation out there.