This will depend on the terms of the trust established under the will. If, under the terms, the trustees are given power to add and/or exclude beneficiaries, then it will be possible for the surviving spouse to change the named beneficiaries.

If we are talking about a will that has not yet been drafted, then it will be possible when drafting it to include a power that allows the trustees to add and/or exclude beneficiaries. If, however, the will is already in existence and the trust is found to be insufficiently flexible, then it should still be possible to change its terms using a codicil, although it may be easier just to create a new will.