Yes, it is a natural progression and often happens automatically as a result of combining SB with sambhavi mudra. The breath will tend to suspend, especially at the brow. Breath retention is an addition to the normal Spinal Breathing practice.
In my book this is perfectly alright. Just be aware that this lengthening of the breath is natural but will also make your practices much more powerful. It is a good thing.
If kumbhaka come naurally then that is fine, quite alright...OK,........GOOOD!!!!!