"Is it possible to connect in series the RF generator (about 900MHz) with a DC power supply (2.5V) to obtain the desired wave form ?"
I hope that you meant it differently from what you've written...
If you only have an AC-coupled signal generator, you need a bias-tee to superimpose the RF signal on the DC signal. For 900MHz you can easy make a bias-tee, since there are many chokes appropriate for this frequency range, or you can even use just a resistor instead of choke.