This can be done using GPL Editor:

1. add new string = name of .srb. Make note of offset

2. add new vertex (point) xyz = position of srb in the 3do. Make note of index number

3. find place in 3do tree for srb. This is the awkward bit.  To avoid clipping the srb needs to be positioned in the correct node of the correct plane. Your 3do will most likely be a load of T-9 nodes - 1st child is above (in front of) the plane, 2nd is on the plane and 3rd is below (behind) the plane. Usually the 3rd node will have another T-9 node. Basically your srb will need to be behind (below) everything that is in front of it and in front of (above) everything that is behind it  ... this is obvious in the real world but you need to consider the structure of a 3do to get it to work in the GPL world. If you select a node and press F2 you'll see all the contents of that node, and this should help you find where in the 3do tree the srb should be. The attached diagram may also help explain how this works if you wanted for example to have a tree behind the two 'walls'. (note that depending on the structure of the object it may actually be impossible to add the the srb without clipping, or at least not without altering the planes first.) If you're lucky you'll find an empty node where you can put the srb ; if not you'll need to cut the node, replace with a T-4 with 2 children and paste back what you cut into the 2nd child and add the srb in the 1st child (or vice versa  ).

4. When you've found the right place/node, right-click and replace node with a T-3 (srb) node.

5. Double-click srb node, type in string offset and vertex number, set scale (in metres) and set unk to 65536. 

6. save 3do