Just to understand your inquiry correctly: You have a int. flight to BOG and onward domestic flight to CTG? Why do you think you can remain airside? There are no domestic flights from the international part of the terminal (domestic and international flights are separated), so upon arrival BOG you follow the crowds for immigration and enter Colombia in BOG. If your bag is checked to BOG only, you pick up the bag at the luggage carousels, exit through customs and proceed upstairs in the landside part of the terminal to the check-in counters + boarding pass and security checks to enter the domestic part of the terminal. If your bags are checked through to CTG, you pick-up the bag at CTG, but you still clear immigration in BOG and proceeds through BP and security checks to get into the domestic airside part.

I suggest that you don't book a dummy ticket just to get access to int. airside part of the terminal. Not worth the hassle and I'm not convinced that the Colombian security staff or officials will approve of such conduct; you don't want to get into troubles in Colombia.