Modify the final FIX message before sending (not header, body or trailer but a new prepend or append string)

Background: The FIX server that I will be using (third party) utilize a custom format to the FIX string. It is non standard to the existing FIX format.

Hi, are there any way to prepend or append into the final FIX format string? ToAdmin() and ToApp() interface only allows modification to existing format, not really modifying the final format of the sent string.

So the current FIX format is this (roughly): [Header][Body][Trailer]

What I want to achieve is this, [Custom Start string][Custom Separator][Header][Body][Trailer][Custom Separator][Custom end string]