There is a usable DShK at the editor > object > weapon > dshk_usable

Paints on floors are a feature of the base game I suppose, so you can copy somebody's paint and paste it to yours.
Everybody can use it without any additional mods.

Other objects or human units are written or drawn by other modders.
Both map makers and subscribers have to enable the source mods.
e.g. This is a collection of a source mod and mod-dependent maps.
https://steamhost.cn/steamcommunity_com/workshop/filedetails/?id=3432875126

If modders make source mods more and map makers also use them more, it would be happy.

It would probably be hard to make it spawn at arbitrary places reliably (if I had to, I'd try to hack something with the "discardWithEntityWhenEmpty" property of RPGs and the Poncho; not sure if it works). But integrating specific places in maps where you can place it should be possible using interactable objects and spawners.
You can also bind specific items to entities. The game does this to allow explosive charges and torches to interact with stuff like walls, fences and generators. Doing the same with a DShK-Backpack and a DShK-Spawnplace should be possible (but the maps have to support it).
Maaaaybe you could bind the DShK-Backpack to Sandbags and use the "discardWithEntityWhenEmpty"-Hack, but it may be hard to get the orientation reliably correct, if it works at all.