Can I keep my existing dwelling and add a second dwelling without subdivision

Yes. You can potentially keep your current house and build another home on the same block without splitting the land into separate blocks. If you want each home to have its own ownership, you may need to create unit titles after construction.

Reference: ACT Government – Advisory Note 19: Block Subdivision versus Unit Title Subdivision in Residential Zones -Block Subdivision vs Unit Title Subdivision (Residential Zones)