You may have to consult a naval architect to do the change properly, if it is possible at all. The other posters are correct in that the rigging
will have to change their attachment points--you'll have to move the spreaders and change the lengths of all the rigging. With longer mast sections between support points (deck-to-spreaders-to-masthead) the NA will have to check if the mast section is big enough in the resulting arrangement to adequately resist buckling. Also, new sections would have to fitted together with internal splices and lots of fasteners. You should not weld on a mast--welding lowers the strength of aluminum
considerably, by as much as 30% in some cases. This is why aluminum
airplanes are riveted together, not welded.
After all that consideration and bother with making such changes, will the cost and bother really be worth it? So you probably aren't saving much in the end by trying to splice new pieces into an old mast.