[quote=@Sodium]
<Snipped quote by Unoedipal>

That primarily depends on how you want to play the mechanics of the Grail's prana supply. Linking to the Grail shouldn't be too difficult at this point, as, after all, this is the 726th time it has manifested - there should be some knowledge by now of how to link rituals into it, but little information on how to draw out of it (other than winning the war). Perform the summoning ritual along with the ritual to funnel the prana and you've got yourself a Servant.
[/quote]

"The 726th Holy Grail" refers to the fact that 725 times in the past, "something" that we call a Holy Grail's materialized. They're not the same device or something, this thing was 100% engineered by the Three Great Families (with tweaks by the Zetsumei). 

The Grail has seven Master slots. When they're filled, they're filled. If you get enough prana and have the right system, it's theoretically possible to summon a Heroic Spirit without the Grail, but they won't be accounted for by the Grail System since they weren't summoned by the Grail.

[quote=@IncredibleBee]
<Snipped quote by Unoedipal>

They're insane cultists, so it's likely that they could develop a ritual that simply forces it to make an extra servant, even if it's at the cost of several/dozen/hundred members' lives.

Or they used their own greater ritual.
[/quote]

You can't just brute-force the Grail into opening another slot, especially if you're just a group of randoms who aren't even related to the Three Families. Not even Zouken could do that.

If you give a good enough justification for why this random cult's managed to develop a ritual that makes something similar to a Servant, then sure, knock yourself out. That said, it won't be related to the Grail and when it dies its juice won't go to the Grail; it'll just be there.