If the virtual machine is running pclos, you can install the guest additions inside it from its Synaptic. However, the version is quite old as it comes from the vbox package which hasn't been built here for a while, and it only contains the most important parts, not every driver the ISO contains. For those two reasons, it's probably better to use the ISO.
If your virtual machine runs anything other than pclos (Windows, FreeBSD, another distribution of Linux...) then you need to use the ISO.