I have found a workaround, which works for me:
Using a draft message (fgwDraft instead of fgwPersonal) you can add attachments.
This will not create a posted task, but a normal task which I have assigned to myself.
set MyNewTask = GroupWise.Account.Calendar.Messages.Add(“GW.MESSAGE.TASK”, fgwDraft)
.FromText = GroupWise.Account.Owner.DisplayName
‘.OnCalendar = FALSE
‘.Priority = fgwNormal
.AssignedDate = today
.StartDate = today
.DueDate = today
‘.Completed = FALSE
.Subject = “New job”
.TaskCategory = “A”
.TaskPriority = 1
set MyNewtask = nothing