The short answer is no.
If you add another hard drive it will become a separate drive letter (usually the one after you CD / DVD drive).
The only way to make a single volume out of two drives is to create a RAID (Redundant Array of Inexpensive Disks) array. if you anly have a 8Gb main drive I'm fairly safe in assuming your PC does not have a RAID facility.
Besides, creating a RAID 0 array, making 1 volume out of two drives, would reduce the capacity of your 40gb drive to 8Gb, so it matches the other drive.
You much better just installing the 40gb drive as a slave (make sure you set the master / slave jumper on the back of the drive to Slave) and using it for your data and applications.
You will need to uninstall the applications from the master drive & reinstall them on the slave, as you can't just move programs.