Of course, and others have said it too, there is singing in way which is the simple following and imitating a melody that you have learned and then there is the capability, like in scat singing, to improvise by singing, the instantaneous creation of music, by means of the vocal instrument, a music which didn’t exist before the moment of its creation.
Most good singers are also good at being singing improvisors although the two things don’t have much to do with one another.
A good singing improviser has to be a good singer, you can have a good singer who is completely at loss of music if asked to improvise.
Improvisation can be learned ( but some find it more difficult than others and some also will never find any improvisation fluency no matter how much they work on it) and it is founded often on interiorizing a canon, a tradition and having the capability to produce variations whilst using recurring elements which will be assembled and deconstructed and reconstructed in an endless variation ( but I am afraid that even the greatest improvisers when subject to analysis reveal the use of modules, patterns and so on).