Are you looking to gain a stiffer suspension? or just raise the vehicle a little higher?
Once you remove the axle from the springs and the bolts from each of the eyes, the springs are no longer under any tension. You can then add a leaf or spacers if you want to.
One way to increase the flexibility/ride qualities of leaf springs is to take them all apart, sand down the rust build up between the springs and put axle grease in between them before sandwiching them back together. This helps them flex like they were originally supposed to do.
I replaced my front struts, have 250,000 on rear leafs they just need a slight arch little work to restore them i dont haul much 4 wheeler and a small pop up thats about it. Alcan leafs has a 1" over factory are $500 ( just leafs I can do install) this is for a Tacoma.
I just swapped in Bilstien shocks in the rear so wouldn't want to buy air shocks, I'm going to keep the thing forever so don't mind putting them on but figured if it was a 1/3 of the price to just get them rearched (haven'e even priced it yet might be as high as new ones), I'd go that route for now, and a little ways down the road but the Alcan leafs in it.
Wheeler's is where I was looking at the leafs, it was there factory replacement HD's made by alcan adds 1" I'm running OME 880's up from on a single cab its about 1" as well. But like you said maybe just throw in an add a leaf, I'm going to see what some of these guy charge to re arch them and go from there.