Generally speaking, yes, however this is not advised.
The definition of Beneficiary in most Family Trust deeds is very broad so it may not ever be necessary to add a Beneficiary. It is also worth noting that if a Beneficiary that could not be contemplated under the wide definition is added, then the addition can create a resettlement which can result in Capital Gains Tax and Stamp Duty being required to be paid for a second time. We suggest you seek independent legal advice before looking to add beneficiaries to an already settled trust.