Some of the boys who are studying engineering (yes, we fashion girls sometimes mix!) are currently messing around building new 'extreme' bicycles for themselves - recumbents, and double-deckers and so on. They often ask me to modify the frames for them, but you have to be very careful. Depending on the make and style of bike, the frame tubes may be joined with lugs that are Sif-brazed. They are very strong and very crack resistant but if you later weld lugs near the joints, you can soft-melt the brazing material, which then oxidises and may later fail.
As well as a fashion designer and welder, now I am becoming a bicycle designer!