Making the RadExpand 5 more comfortable

[tdnfgk]

I have a fused spine and find it difficult to pedal in an upright position. I need to be more semi-recumbent. The attached photo shows my other bike with a saddle seat and backrest, further behind the sprocket. I know I can't achieve this offset on my RadExpand 5, but do you think a seat post with a big offset coupled with a saddle seat w/backrest would help me? Any recommendations?