Converting bits and bytes

I'm implementing a specification to send and receive data from an external device. The 'lowest' layer which accepts a byte array is done but I'm having trouble converting all the variables from the upper layer to a byte array.

For example, there is an integer field that is 4 bits long. Let's say I have the following variables:

varone(2) as byte
vartwo as byte
varthree(4) as bitarray 'an integer field
varfour(3) as byte
varfive as byte
varsix(4) as bitarray 'a second integer field

How do I take all these variables together (preserving the sequence) to a single byte array?