Hi Pedro,
You would have to remodel your manipulator such that you have a series of links between the actual manipulator base and the fixed base. These series of links will be each attached (in sequence) by prismatic joints in the X, Y, and Z directions of the fixed base frame. You can make use of the add subtree function to add your manipulator to the moving base rigid body tree (i.e., the series of links with prismatic joints).
Here is the documentation link for addSubtree - https://www.mathworks.com/help/robotics/ref/rigidbodytree.addsubtree.html
Hope this helps,
Karsh
