Is it possible to use an existing field into a matrix? I'm trying to reuse a dropdown field inside of a matrix. I've already created a field which has hundreds of dropdown values. I want to be able to select from this dropdown field for each item of a matrix. So instead of having to re create this field for each matrix block, is there a way I can copy/paste a field to a matrix field? 

Thanks in advance!
You can use the neo Plugin or you can copy and paste the field settings manually from one field to your matrix field in your database interface. In your craft_fields table there is a column settings you can search for your field handle and copy the settings to your new field. 

You'll not be able to do this without a custom plugin or copy pasting the values via SQL in any way