There is certainly nothing to prevent parish councils from receiving such donations, so there is no reason why they can't be proactive in seeking them. The difference between local councils and national charities is that the charities have an ongoing programme of work and need money every year for a specified purpose, whereas local councils might have a short term fundraising target (a new village hall being an obvious example), but the timescale would normally be limited to, say, five years, after which the money would no longer be required. Legacies are more appropriate for the former.
Having said that, if you can find a specific long-term project, such as establishing a parish charity if you don't already have one, legacies would be an important part of the mix. Asking people to make a donation to a parish council many years in the future for no specific purpose is unlikely to yield a positive response.