Custom virtual gel ladders can be added in R7 onwards. To add a custom virtual gel ladder to Geneious Prime open the virtual gel view tab for a sequence. Next to the dropdown box listing the available ladders click the "?" button. Clicking this button will open a dialog window showing the currently stored virtual gel ladders. You can edit this window to add new ladders or modify the existing ladders.
Why do I keep receiving the message "Could not parse ladder lilne(s)"?
Sorry, I realised I replied to your support case, but never posted the solution to the forum. The sizes in the ladder need to be in order from smallest to largest value.