For extract, I'd just sub wheat DME and not worry about the GNOs. The problem with adding any hops into the beer before souring (and you need the heat to extract some of the bitterness and aroma/flavor compounds), is that the hops will stop the lacto dead in their tracks. So it has to be soured, brought up to whirlpool temperatures, and then the hops added.
However, you can also blend. Do your brew day as normal, but draw out half the batch and chill/pitch lacto. The other half, add your hops and either hopburst it or whirlpool, with the idea of ending up with about double the IBU as the final blend, assuming you've split it 50-50. Chill this portion, and pitch yeast.
After a few days, rack the soured half onto the hopped/fermented half, and let it go. Might take a couple tries to get the right balance between sour and hoppy, but that's the general idea.
I really can't say how much lemongrass, but I would start with less than you think it'll take, sample after fermentation, and add more if you think it's needed. For the passionfruit, I would start at 0.5# per gallon. You can always add more, but it's hard to take it back out. Unless the fruit you're adding contains a lot of acidity (and the beer is already sour, so not much of a concern), it's pretty hard to over-do fruit additions. At 0.5#/gallon, chances are you'll want more, but you might want to try it and adjust as necessary.