To copy and paste a mask from Illustrator to After Effects there are a couple of things you need to do;
1) In Illustrator. Go to Preferences > File Handling and Clipboard
2) Make sure the Preserve Paths option is selected
3) Select the Path and copy it
4) In AE, create a solid (Cmd/ Ctrl – Y) the size of your comp
5) With it selected, go to Edit > Paste. The mask should be visible now on your layer. (If not, make sure the Mask Display toggle is on in the Comp panel)
6) If you need to adjust it, Hit Command (ctrl) – T to Free Transform it.
7) Apply the Stroke effect, make sure to select the All Masks option.
Hope this helps.
cheers,
Angie
Angie Taylor animation & illustration for television, film, web and devices
https://www.angietaylor.co.uk
Twitter: theangietaylor
Linkedin: theangietaylor
https://www.facebook.com/pages/Angie-Taylor/118378194869002