adding pad holders to existing pack

You can put on 1" thick tabs of cuben tape where you want the cord to go and put small grommets/eyelets through that. Then you can run your shockcord through. I think that would be the easiest/cleanest way to do it.

Yes, certainly possible.

To use a pad as a frame requires some different streses on the pad. I use a 5 layer nightlight pad (or Nunatak Luna pad,) meshed together so the bumps act as an interlocking layer. This will keep all stress fairly well contained.

Anyway, two fairly full sleeves are wanted. Not quite half way, up and down, and with enough stretch to handle the thickness without being overly stressed. These can be double stitched to the back panel. The corners cause the most problems inserting the pad, so removing about a 2" triangle works pretty well.

This will stiffen the pad against vertical stress. This lets the pad/pad keeper/pack combination to easily carry 25-30 pounds of load with an otherwise frameless pack.

Using cord and eyelets, the loading drops to about 15-20 pounds. I usually carry a Murmur, so, this is about the maximum load for that pack. The limitation is the suspension, not the construction.

With other packs, there is always the option of a burrito roll. Roll up the full length pad and insert it first into your pack. This will usually add a limited amount of frame support, though. Usually around 20 pounds.

If you do a custom build, you can tailor you pad in 3 layers by adding a pair of side sleeves. This uses the corner strength to stiffen a pack without the loss of hip movement. It can still follow your hip forward and backward as you walk. A couple 3/4" straps hold it in place. This will add a lot. About 45-50 pounds maximum before the pads will fail to support the load. All done by reusing a component you carry anyway in a different role. I am a big fan of these types of mods.

It all depends on what pack you’re doing this to really.

Another pretty simple and inexpensive solution would be to buy a seam ripper and some of these and cut the webbing into two inch strips. Flip pack inside out, measure twice/three/nine times and use the seam ripper to open that main backpanel seam 1″ wide (or width of those straps) and take the 2″ long strips, fold them in half, and sew them back into the main seam (doesn’t require a machine, a hand stitch would be just fine). All you’ll need is some elastic cord and you’re good to go.

The outcome would look something like this.

Photo Credit: Hendrik Morkel

Joe at Z-Packs has these style pad holsters:

Easy enough to retrofit to your pack the ones from Joe or make similar items for a complete MYOG project.