load your schematics in mcedit, and export them as a structure. (I believe mcedit could export to structure files)
run minecraft 1.12.2 and create a blank superflat world for converting your structures
Place all of the structures, keep the structure blocks
upgrade world to 1.13 and re-save all of the structures
add the new structures to desired 1.13 world and load the structures in game
*Double check that the blocks haven't changed after conversion.
*If you can't get mcedit to export the schmatic file to structure files, you could place them in a 1.12.2 world in mcedit, but you would need to define the bounds of each structure
*You're limited to a 32x32x32 for each structure block, and this can easily become very scrupulous.
Not sure if there are easier ways of accomplishing this, but this is what I had to do with converting all of my 20ish structures since the 1.13 snapshot I was in couldn't load 1.12.2 structures. Who knows, maybe they added the ability for 1.13 to directly load 1.12 structures. I Would try that first just to see.
It seems to be possible through the use of the WorldEdit plugin via Spigot, albeit experimental as of right now.
You can find a WorldEdit beta release for 1.13 on the bukkit website, create a localhost server and import the schematic to your world there. Again, it's currently in an experimental state so I'd only recommend this if you're okay with potential bugs.