Yes, all previous relevant service of 7.5 hours per week or over is reckonable for superannuation purposes.
Any nurse/midwife who works over and above their contracted hours up to 37.5 hours per week is entitled to have this time recorded for superannuation purposes. (Circular 23/2005 Part D)
The contributions you will have to pay in respect of such service are calculated on your current rate of pay. You may pay this amount in a single lump sum payment, or pay extra contributions in addition to your standard contributions.