Is there a way to insert values in tabes that are inside of tables?

Of course. Things inside tables are just normal values, so you can do anything to them that you can do to any value.

local cframes = t.cframes
table.insert( cframes, CFrame.new() )

-- or, more simply

table.insert( t.cframes, CFrame.new() )

One of the main goals of a programming language is to provide consistency. With only a few exceptions, code that works in one place in Lua works anywhere in Lua.