As far as I understand, the viral nature of the GPL does not allow this.
If you copy and paste functions into your code, then those functions are still under the GPL, and your code is still under the BSD.
Anybody could then copy and paste your BSD code into their proprietary software, and only have to conform to the BSD licence's requirements.
The main issue with this is that it when you mix code from various sources in the same file, it can be very difficult for readers to know what licenses each section of code has. For that reason, when I need to include code which has a different license or copyright from the main work, then I put it in its own file with its own copyright/license header.