Yes, simply open the second project. Drag the sequence from that project into the Timeline panel, but towards the top, so it shows a blue area along the top. Let go, you'll have two stacked timeline panels.
Then simply select the sequence from the bottom panel, and add it to the one in the top panel.
You can open the existing project with your previous project still open.
Select all in the previous project and Edit/Copy.
Then select your current timeline and Edit/Paste.
Also, if you use the keyboard shortcut to paste which is Command V on Mac or Control V on Windows you can press shift with those keyboard shortcuts to do a paste/insert to whereever your timeline indicator is.