Umaro In WoL

how would one go about making Umaro available in the World of Light?

I know in the World of Ruin you find him in his cave. Would it be simplest and as easy as placing him on a map somewhere and after you talk to him, he joins?

Thanks in advance

It's just a matter of tweaking the event in order to set the appropriate event bits so he can be visible on the airship and in the menus. Also, you would need to remove or disable the Narshe cave events so it makes sense.

Well, why not in a battle
"Umaro was originally supposed to be Random Encountered on the World Map and could only be caught with a bait. Hidden game data also suggests that Umaro was at some point intended to be able to be encountered in the World of Balance, as an unused battle complete with stats and a simple AI script exists within the game's data."
Source

You would be able to get him to join the party in that way, as long as you set the appropriate event bits, such as the bits that make Umaro available in the shop menus and party select screen.

To actually place him in your party, you can just use 3F 0D 01, which places Umaro (character 13, or $0D) in party 1.

One issue that you may have, though, is later events where the game needs to check whether he is in your party or not. Many events essentially run down a list of characters that may be in your party, in order to make the most relevant character say the dialogue. A good example of that would be that event in Zozo where you find Terra there for the first time. The game checks if more verbose characters like Locke or Edgar are in the party first, and has them say the dialogue if they are. It checks for Gau last, so unless the only character you brought is Gau, he's not going to say anything. The reason why this is relevant is because, if the player were to bring Umaro to an event like this, and if he's the only character in the party, the game won't check for him at all, so important lines of dialogue may be missing.

The address for that portion of the event dump is CA/9B20, so you can take a look at that if my explanation is unclear.
Umaro should actually even appear on the Blackjack if you set the appropriate event bit.