I dont use Sage payroll but use sage accounting software and understand it might be the same process. Worth trying the help menu and just search for 'import' or you could go via File---> import - the system will take you through what it needs and you can see the headings required before you need to attach a document.
Make sure you back ut up before trying anything anyway to be on the safe side.
Might be ok to import from excel but might need to covert it to a .csv file
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Joanne
Winner of Bookkeeper of the Year 2015, 2016 & 2017
Thoughts are my own/not to be regarded as official advice,which should be sought from a suitably qualified Accountant.
You should check out answers with reference to the legal position