get all checked checkbox values from a powermail field in typoscript

Question

I’m looking for a way to get all checked checkbox-values from a field in a powermail form. I can access single values from the array in my typoscript template with data = GP:tx_powermail_pi1|field|marker|index. Is there a way to interate over the array when I don't know the number of checkboxes (dynamically filled) or to serialize the array? I am not experienced with typoscript or powermail but allready tried to find a solution for quite some time.

What i am trying to achieve ist to have a multiselect field for people to send the mail to that gets populated from fe_users.

The relevant code from the typoscript template that overwrites the receiver is:

plugin.tx_powermail {
    settings {
        setup {
            receiver {
                enable = {$plugin.tx_powermail.settings.receiver.enable}
                overwrite {
                    email = COA
                    email {
                        10 = CONTENT
                        10 {
                            table = fe_users
                            select {
                                pidInList = 8689
                                where {
                                # UID of the fe_users record is given in field with marker {fragegehtan}
                                    data = GP:tx_powermail_pi1|field|fragegehtan|0

                                    wrap = uid=|
                                    intval = 1
                                }
                            }
                            renderObj = COA
                            renderObj {
                                10 = TEXT
                                10.field = email

                                stdWrap.wrap = |,
                            }
                        }
                    }
                } 
            }
        }                  
    }
}

This is supposed to create a comma separated list of uids of all selected fe_users. Ist there any advice how to do it?