If you mean extensions for objects, then yes you can. Below is some very useful information on it -
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no there is currently nothing in Qlik Sense like a Document Extension in QlikView.
The concept of extensions in Qlik Sense is very similar to the "Object Extension" concept in QlikView, so they can be seen as visualization objects, can be used multiple times per app/sheet whereas a document extension was only loaded once per document/app in QlikView.