Is it possible to insert a CHP running on bio oil in HOMER, which contributes to the renewable fraction?

Yes, it is possible. You should add a generator that uses a biofuel (like "biogas" for example). You can define your own biofuel in the library view. Go to fuels, select a built-in fuel, and click "Copy". Be sure to select the option "Bio Fuel Source" for this fuel. Once you have created the new fuel, it will appear in the fuels drop-down in the generator menu. Be sure to also change the generator's fuel curve to reflect consumption of the bio oil. To model CHP, set the generator's "heat recovery ratio" to a value other than zero.

You can then add a biomass resource that is the raw biomass used to produce the bio oil. A bio fueled generator is considered part of the renewable fraction. I have attached a very simple sample file in case it is helpful. HOMER's built-in help manual can help explain the meaning of the different parameters of the fuel, biomass resource, and the generator.