well… it can be done IMO, but there’s no one-click solution.
I don’t know the inners of Lemmy too much, but at first, without any reading up, my approch would be first create a Telegram bot and invite it to every channel you want to migrate to Lemmy as a Community or post in a community. Not sure how would you like to lay out channels in Lemmy, but let say this time it will be community/channel and one message in channel will be a post in Lemmy, and threads of messages/first replies of that message would be lemmy comments.
but either way, according to any logic, this Telegram bot would export all messages in a structured way; json or straight to sql. With this, Telegram part done, but for all the info you need to setup your Lemmy.
on your lemmy, the people used your Telegram channels should make an account. Make note of their IDs, your Telegram bot should do the mappings. if it’s irrelevant, link everything to a dummy/bot user
now this is the part I’m not sure of if possible, but the rest should be just the matter of merging your export into Lemmy db, to create communities, posts, comments. Probably media uploads are neccessary too. and you should see your Lemmy populated. yonly thing is, there would be no scores for the imported data, but users should have posts and comments.