Hi Jack, I have a Vermont American version of that machine, have had it 40 years, but used it very little after I bought a lathe. I do intend to use it again sometime.
You will in theory be able to do what you have in mind (based on your images), but bear the following in mind:
1. John Lennon's idea for joining the tubes is good, but you will not be able to use the tube supports or the grub screws he suggests, as they will block the travel of the router carriage. Either use much longer internal connectors (if you can find a stiff push-fit), or replace the tubes with longer ones (perhaps of greater wall thickness, so you do not get sagging in the middle, if you are using a heavier router). Theoretically, you need not confine yourself to double the between-centres length either - why not 2 1/2 or 3? But I cannot speak to the practicality.
2. I don't know about drawing boards, but the cabling system is designed to move the router carriage laterally in a fixed ratio to a number of turns, to get the spiralling effect. To achieve this, the cable winds and unwinds around a plastic drum at the "headstock" end. The cable is fixed to the drum at its midpoint. I very much doubt that there is enough room on the drum for a much longer cable. Extending the cable by splicing at the tensioning spring would therefore not increase the router carriage travel.
3. All is not lost:
a. The carriage can be disconnected from the cable. This is how one turns round posts from square stock, moving the router (on the carriage) along by hand, flattening the corners first. Thereafter with someone else rotating the handle at the "headstock" - perhaps you could even fit a small geared motor with foot control. Caution: I am not sure whether the finish of the turning will be to your requirements. Extensive sanding may be necessary to remove ridges left by the bit.
b. The tailstock can be offset, to create a linearly-tapered column.
b. The headstock has an indexing feature, which would allow you to rotate the workpiece in fixed increments, and lock it in place, so that you could slide the router and carriage by hand, with a veining bit, to do the reeding/fluting. ~~Be aware that the point of the bit does not travel straight up and down - it travels in an arc, centred on the rear tube. The slight asymmetry of the sides of a V-groove may not matter, unless the cut is deep. Your client probably will not notice, but you will.
c. If you first make a plywood or hardboard template of the cross-sectional view of the posts or columns, you can make fairly exact replica posts, as the router carriage has a follower which rides on the edge of the template (which attaches to the front of the frame.The template would be necessary if you plan on fluting a non-linearly tapered column, otherwise the router will cut to the same depth from the centre of the workpiece (not the surface), and ruin your work. If you offset the tailstock, as in b. above, the template is not necessary for fluting.
I have limited myself to making pieces which are glued together, but in principle your idea should work - a bit laborious, but not many people have access to a lathe with a bed length of the size you want.
I had to drill at least one mounting hole, as the only router I had when I bought the crafter, was a Hitachi M8, which had a base with no holes at all.
Do you have the manual for that machine? I have a ratty Craftsman pdf at hand, and could probably find my original somewhere.