Yes, I've done this many times in one of two ways.
Wood support as core
Build wooden posts, two per side. Then connect them sideways about 1/2-way so you have lateral box. These cross-pieces locate tops and bottom of posts. On bottom of posts, carve U-channel to wrap around frame. Attach metal-brackets as necessary to hold firmly to frame. Finally, wrap 2-3 layers of fibreglass around wooden structure to add strength and water-proofing.
Weld metal support
For race bikes where strength and light-weight are in conflict, I weld metal bridge onto frame to support tail-section. bolts from top through fibreglass onto metal-bridge (welded nuts underneath metal bridge). Lay down seat foam, making sure to cut holes to access mounting bolts. Ends up about 1/2 weight of wood structure.