There are many solutions of varying complexity, depending on what you need.
100 isn't a huge amount and may fit on a single atlas page. If not, you may be able to put some parts on a separate atlas page (using folders). Eg, if you can put all weapons on their own atlas page, rendering your skeleton would draw from one atlas page, then draw the weapon from a second atlas page, then draw the rest of the skeleton from the first atlas page. That's 3 texture changes to render the skeleton, but that may be acceptable.
Otherwise, you can customize the AttachmentLoader, see here:
http://esotericsoftware.com/spine-loading-skeleton-data#AttachmentLoader
The AtlasAttachmentLoader expects to find all regions loaded from a texture atlas at the time the skeleton data is loaded. You can provide your own that doesn't have that requirement. The attachments are all loaded with the skeleton data, but not configured with a texture region. Before drawing, you need to configure them. You'd outfit your character, pack at runtime only the images the character uses into an atlas, then use that atlas. If you are using Unity, some of this is done for you (see the examples). I described some of how it works here (and elsewhere many times):
https://esotericsoftware.com/forum/d/23863-swap-texture-atlas-scale-at-runtime/2
If you have many images, like thousands, setting them up in Spine can be tedious. Instead you can use a template attachment and replace it at runtime with the actual image you want to use. See:
http://esotericsoftware.com/spine-runtime-skins
