The quick answer is no. It is not possible to add user defined toolbars to Trilogy.
I suppose you would like the button to activate some kind of automation/script. If that is the case, as a possible solution it is possible to "bind" function keys to do this. Under the Script menu there is a "Binding.." menu. That will enable you to bind the F# key and other combination to active script. That should provide immediate assistance.
Most Trilogy DFM related users are moving to Valor NPI. If your organization is also performing assembly programing, then a combination of Valor NPI and Valor MSS Process Preparation is the combined solution.
May I suggest that you consider visiting the Mentor IDEA site for Valor NPI located at https://mentorideas.brightidea.com/valor and add this as an enhancement request for a future version of Valor NPI?
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