Yes, you can indeed implement and use an Expert Advisor, whether you use a third-party one or your own.

However, we also must inform you that AvaTrade cannot assume responsibility nor be held liable for any issue arising from or encountered with any Expert Advisor you implement on your account; it is solely the responsibility of the account holder.

We take no responsibility for the functioning, effectiveness, maintenance or results of any external or third-party EAs or any customer-created EAs.