Panoramas require a bit different workflow than normal photo editing. This is due to the requirement for multiple source files. The tool of choice for panoramas is Autopano Pro but latest version of digiKam should do well and fits zero budget. Following list describes the steps required:
Tag the photos belonging to a panorama in KPhotoAlbum appropriately. This is done mainly to avoid discarding images by accident, when sorting out the garbage among the photos taken. You probably want also to stack the set of source images at this point.
You have at least two simple options for next step:
You can let Autopano Pro find the panoramas automatically from the photo directory.
Or send the proper images to Autopano Pro on command line to speed things up. This is handy if the image directory contains a lot of images, especially RAW ones. This requires one to fix the script that starts Autopano Pro to accept input and to create a desktop file or invoking external program with own command line.
If it happens that the resulting panorama is missing a small part, you can try to craft it e.g. with the Gimp. Gimp works usually quite well if there is enough "structure" to be cloned but there is absolutely no guarantee you will get usable results.
Save the new image somewhere under KPhotoAlbum's image directory and run → . This should notice the new panorama and include it in the picture gallery.
If you have set the AutoStacking of new images properly and name of the new panorama matches the rules for this, the panorama should be on top of the stack. Otherwise sort the images by date, so the new panorama ends up at by the source images and stack them manually. (Autopano Pro seems to write the EXIF info of the first image into the panorama.)
You can still add tagging at this point to the panorama or you might include all the source images also with Ctrl+2.