The acquired businesses will be integrated into STM subsidiary Options UK.
The announcement said that it will be business as usual for clients and employees of both businesses.
Options UK was formerly known as Carey Pensions and rebranded earlier this year when it announced it had further acquisitions planned for the SIPP and pensions sector.
STM will pay an initial instalment of £1.4m, with the remaining £1.5m to be paid subject to the meeting of performance targets.
Berkeley Burke (Financial Services) and Berkeley Burke Employee Benefit Consultants are independent of the Berkeley Burke SIPP business which entered administration in September 2019 before being sold to Harley Pensions in February 2020.
Each of the businesses has annual revenue of approximately £1m.
Alan Kentish, CEO of STM Group Plc, said: “We are really pleased to get this acquisition under our belt. It has been a long time coming but we needed to find the right acquisition for us. The SSAS business fits nicely within our existing operations and complement our continued expansion in the UK pension scene. The Employer Benefit Consultancy business builds on our desire to diversify and become stronger in marketplace.”