There is a plug-in available here which I think will do what you need: https://forum.audacityteam.org/t/extend-silences/34612/1
Read the full post for instructions and a full description of the effect.
There is a plug-in available here which I think will do what you need: https://forum.audacityteam.org/t/extend-silences/34612/1
Read the full post for instructions and a full description of the effect.