Yes. Even though Open Collective lets you directly raise money on the platform (for a 5% fee + credit card fees), you don't have to use that feature. You can use any other crowdfunding platform out there and then add the proceeds to your open collective. In that case there is no fee (well, just the fees of the crowdfunding platform that you would be using). That's what XR Hackney has been doing. They use Chuffed.org for their fundraising.