Wekan has API and Outgoing Webhooks, but they are different than Trello:

https://github.com/wekan/wekan/wiki/Features#api

Is it possible to use Trello extensions and integrate them with only Trello API ? And is it allowed to use Trello extensions outside Trello ? Or would they require separate integration to Wekan ?

For example on Trello templates can not be used outside Trello:

https://trello.com/inspiration

For general integration (similar to Zapier) there is plan to use Open Source tool Huginn:

https://github.com/wekan/wekan/issues/1160

https://github.com/huginn/huginn