I have just tried this myself, it’s a good solution but not perfect yet, because some of the units do not work and you have to add them yourself. I wish the devs would just give us the option to do this more easily, I want to play all the old custom campaigns from aoe heaven in ROR please!
I really wish they would remake aoe1 buildings to fit the new aoe2 set… And if one is conservative there’s still aoe de… It’s really evident in game that the two things don’t merge well, when you see a tile of gold and an house.
I’ve done it plenty of times, just as @Apocalypso4826 said. Copy from AoK, TC, HD or whatever to the 2DE scenario folder and load them from the 2DE menu
Here comes the fun part! In the AoE2DE Editor, under the Terrain tab, use the Map Copy tool as seen in the above image, copy the entire map (simply drag from the top-left corner to the bottom-right corner and select the whole map). Now exit the AoE2DE editor and switch to Return of Rome and open the RoR Editor, create a blank scenario, and make sure the map size is big enough to accomodate the map you just copied from AoE2DE. Go to the Terrain tab and use the Map copy tool to paste the entire map on a blank scenario, and there you have it!