Yes you can! If you'd like to use a particular project as a private template (i.e. for your own use only), see the instructions here.
On the other hand, if you would like to share your custom template with the community, you can upload it as a document on Overleaf and then choose 'Journals & Services' from the publish menu in the editor.
You can add additional authors and a short description to your template, and should ensure the box marked "Let people use this document as a template." is checked before submitting.
Your submission will then be sent to the Overleaf team for approval. Once approved, your submission will listed with the other templates in our online gallery, and available for use as a template for creating new documents.