I have done short 35mm add ons as well. I do this for my 35mm stuff as quality control.
I take a film, set up a grey card, a color control card from a Kodak color darkroom dataguide, and persuade my wife to stand in front of the house on a cloud free day holding said cards.
I meter, set camera on matching manual exposure, bolt tripod tight and wind off about 72 exposures over a couple of rolls as a constant.
Then for each batch of c-41 I mix (I home brew this from base chemicals) I snip another 4" or so off one of these test rolls and add it onto a roll I am developing. For a long while I was/still am working my way thorough 100 rolls of Agfa Vista in 27 exposure lengths, so there is lots of room.
I have used masking tape, but recommend splurging and buying film splicing tape. Mine is called Blue Max. It sticks to film base very well, but is not too sticky on anything else. My roll was $20, but I use it 1" at a time, so I have a lifetime supply.
Practice with scrap films (see, there I go again) Keep track on where the the end of the film is as you load, and before it gets to the ball bearing gate lay the film tail down further back on the reel on the outside of the spiral, and then lay the new film to overlap by a 1/4" or so. Using the reel as an alignment template helps the film to go on in line in the dark. If it gets half way in and binds, you can undo and re-stick the take if using blue max. I fold a little corner over when prepping the tape in the light to do this. With the corner to tug with a finger nail you can make ginger adjustments. If you get really stuck, cup the film like loading a stianelss steel reel, and wiggle it past the ball bearing areas where it tends to snag when loading without the ball bearings.
Trim the tag along film so the corners are chamfered, and the cut is square. Square corners tend to snag if not perfectly aligned.
If you are not filling the reel, load the first short film, and push it along the spirals until it hits the core. Then start to load the second short film. There will be a length of spiral empty betwen the two films, and no need to tape on. I tape on because a 4" long film chip ends up coming off the reel and scratching itself and the films in with it if it is not tacked onto the reel in front of it. I have even taped onto and then had the chip dangle betwen the reel outside and the tank inside when I have a bit of a long bulk loaded '36' exposure film to process. The edges ge scratched, but I can still read the density of the grey card and colour of the test patches with the densitometer to judge if the fresh bathc of chems is similar to that which came before it..
