In the cosy idealistic world of the bean counter, conversions have fantastic appeal. In the real world, however, it all ends in tears and lots of finger pointing.
When an airframe, which is designed to a specific requirement to do a specific function, undergoes major surgery to make it do something else, the in service dispatch reliability becomes poor. The mechanical wet-nursing that they require is very costly.
Converted aircraft are seldom as capable after conversion as they were when they first rolled off the assembly line.
CR2
Can't even carry its own engine...
Not sure why you think this is such a desireable ability. I take it you're talking about fifth pod. I have only ever done it once on B74F. Generally speaking (and the ETOPS folks will support this) engines are so reliable nower days that a fifth pod requirement becomes obsolete. I'm sure that the A380 team have looked at this.
If I were in the market for a long haul freighter with wide bodied capacity I doubt whether I'd go for a B744F especially an SF. The capacity is too big and puts you at the perils of the consolidators. I think a better bet would be an A330F made pureley for the job.
FEBA