Some code that may help someone else in the same situation :-)

string[] rolesArray;

            // get currently logged in user
            var member = System.Web.Security.Membership.GetUser();

            rolesArray = Roles.GetRolesForUser();

            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            sb.Append("Roles for " + member.UserName);
            foreach (string role in rolesArray)
            {
                sb.Append(role);
            }

            var profile = ProfileBase.Create(member.UserName);
            profile.SetPropertyValue("datePlanExpires", DateTime.Now.AddDays(30));
            profile["interests"] = sb.ToString();
            profile.Save();