I completely understand this request.
However Basic4android data types are based on Java data types which are signed. Adding a new "primitive" type such as an unsigned byte is not impossible however it may be problematic.
Once you fully understand how to work with the signed byte you will see that it doesn't really matter whether it is signed or not.
You can create a simple method that converts a byte to an unsigned value:
Dim Unsigned(b As Byte) As Int
Return Bit.And(0xFF, b)
End Sub
Dim b As Byte = 200
Log(Unsigned(b)) 'prints 200
Only when you try to "see" the value you will see that it is signed and not unsigned.