The teenager has agreed on a switch which will see him turn out for the Hampshire Premier League side when his Woking under-23s commitments allow.
Gosport-based Sims, 17, is captain of the Cardinals development team, but has also dual-signed with the Coptermen for the forthcoming campaign.
Fleetlands boss Rich Bessey, who came to know of Sims through his work coaching within the club's youth section, believes the versatile prospect can have a real impact at Lederle Lane.
And he will get his first chance to feature for Fleetlands in their home friendly against Bosham on Saturday.
Bessey said: ‘We’ve got a relationship where we will play second to them (Woking) - I think he plays on a Thursday so it shouldn’t clash with us.
‘But there will be games where he won’t be able to be with us because he’s got to train or play with them.
‘We’ve got to look after him, we want to give him some experience in men’s football, he’s obviously playing in the under-23s which is fantastic for him and they want to see him in the first team squad at Woking.
‘We’re grateful for the opportunity and he’s a real good asset. He’s 17 and he’s a real strong contender for getting in our first team already.’
Sims becomes Fleetlands' third summer signing, with former Fareham man Stu Maunder joining and Jake Alford returning to the club.
Alford struck 20 times in 31 league and cup appearances for Fleetlands in 2017/18 and 10 times in 29 outings the season after. He netted 14 goals in 22 appearances for Locks Heath in 2019/20 and spent last term at Wessex Leaguers Petersfield.
Bessey, though, has lost Matt Andrews and Bryn McKie (both retiring) along with young gun Jack North - son of Pompey Academy coach Shaun - who has moved to Sussex side Selsey.
Preparation for the 2021-22 season started at Whitchurch on Tuesday, with Bessey's troops claiming a 1-0 friendly win thanks to Jamie Wrapson's strike.
The Fleetlands boss was buoyant as his team started pre-season by beating division higher opposition - even though they were missing several first team squad members.
Bessey said: ‘There were some reserves and six trialists in the side which was fantastic.
‘There was no Luke Heard and no Aidan Smallbones, who were both my centre-halves last season, no Matt Short - he’s our goalkeeper who won players’ player of the year . We had no Sam Martin, who won manager’s player of the year.
‘We had no Stu Maunder, so we had a lot missing but we still delivered.’