Is there a way to add a custom map(Item) to my server?

You mean: Image on the map?

Try this, screenshot the map you need and edit it to suit (Aternos probably has limit on image size so making it thumbnail size would help) then on https://aternos.org/options/ click the picture to change it, this would still only give you a static image though, to have one that is "live" (self updating/adapting) you would have a website showing the map of the game, a useless spare PC to run code on, code to automatically screenshot the website, edit it and upload it daily. That's a lot of work for what I think you're asking though, I'd just stick with a single edited screenshot

Nirmarea he said Map (item in minecraft), not "world" map

You can install this plugin: https://aternos.org/addons/a/spigot/26585, at least that's my opinion because that's the only plugin it supports 1.12, but you can use link to upload map instead of uploading via your computer