Holt scored 78 goals in 168 appearances for Norwich during a four-season spell, and helped the Canaries to back-to-back promotions.
Now, the 36-year-old is set for a stint in non-league after being released by Hibernian at the end of the 2016/17 campaign.
He has been joined by Lappin, who made 126 appearances for Norwich between 2007 and 2013.
With the lofty aim of promotion in mind, Culverhouse has so far led King's Lynn to seven wins and a draw from their opening 10 matches.
Holt and Lappin could make their debuts against Hitchin on Tuesday night.
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