Can I create my own maps?

Yes! All you need to do is create all the rooms, save them as structure blocks and list their names in the dungeon datapack json. Though there are a lot of details involved

How to create your own Dungeon Map?

You must create a json for your arena:

copy an existing json from the mod jar
make sure each type has at least 1 room
Structure files must be max 16x[height]x16, height can be maximum possible

To actually spawn content in these maps, you need use command blocks with data 'tags' or ids. Basically the Mod detects the text inside command blocks in the Dungeon Realm, and spawns content accordingly when a player is nearby. For example, the most common id: 'mob' spawns about 1-2 random mobs at the location.

'league_back'Sets block at location: library_of_exile:teleport_back
'mob'Spawns a few mobs
'chest'Always spawns a chest
'map_reward'This chest is a bit special, when you enter the reward room it will only spawn x amount of chests depending on your map finish rarity/score.
'elite_mob_horde'Spawns a lot of mobs. [With Mine and Slash installed, the mobs are higher rarity]
'map_teleporter'Sets block at location: library_of_exile:teleport_back
'teleport_back_block'Sets block at location: library_of_exile:teleport_back
'elite_mob'Spawns a few mobs, [With Mine and Slash installed, the mobs are higher rarity]
'uber_boss_altar'Sets block at location: dungeon_realm:uber_boss_altar
'league_spawn_pos'In case your boss arena, uber arena or reward room have very weird spawn positions, you can force players to teleport to this position instead.
'map_boss'Spawns the map boss, 1 of these is required per every Boss Arena
'chance_chest'Has 25% chance to spawn the chest, but only spawns it once inside a single room. Use this to randomize chest positions easily by placing a few of these in different spots
'mob_horde'Spawns a lot of mobs

To create a map, you pretty much only need to know about: mob, mob_horde, chest and chance_chest!

That's it, have fun creating custom Dungeon Maps!