Can I charge an ebike from another ebike battery? I have a fiido d2s, and I love it. I'm not sure if I could upgrade to a bigger bike, because it fits so well in the back of my car and public transport. Anyway, it doesn't have a detachable/replaceable battery.

I've used this: https://www.amazon.co.uk/gp/product/B07TG8WB27/ref=ppx_yo_dt_b_asin_title_o09_s00 and the charger that came with it while out and about to extend my range. It basically takes DC power, converts to AC, and then converts back to DC. I believe the bike's battery is about 280 Wh and this gives me another 80 after some loss, give or take.

But I'd really love to double or 2.5x my range. I've looked up tutorials like this one: 
                
            , which end up with a battery that can charge a scooter (or ebike) using the DC input that comes with the bike for charging.

But getting enough batteries alone (aside from other parts and me probably screwing things up) costs almost as much as one of these: https://www.ebay.co.uk/itm/Bicycle-Ebike-lithium-li-ion-Battery-24V-36V-48V-10Ah-20Ah-1500W-350W-500W-1000W/174033645919 (not almost as much, but also not insignificant).

Some of the ebike batteries have 36v, 2a discharges which is the same as the charger that came with my bike. Is there any way to take those and charge my bike's internal battery?

Edit: I appreciate all the suggestions. For the record, I know that charging and riding at the same time will work. I do it with my 100 Wh battery pack already. And I know I use more electricity riding the bike than can be pumped into it. But I think if I could charge at the maximum (36v, 2amp) while riding the bike, I could extend the continuous riding time from 3 to 5 hours and maybe the total riding time to 7 or 8. I’m sure I could do this if I just bought a 300+ Watt-hour battery pack. Something like this: https://www.amazon.co.uk/dp/B07DVGYDL5/ref=cm_sw_r_cp_api_i_vTlEEbJ1GRQ9R

But I can get a lot more watt-hours for a lower price and in a friendlier shape with the ebike batteries, vs that thing.

I’m wondering more if there’s a simple process to connect the discharge port of the ebike battery to the dc input of the bike. Or does it need some sort of conversion?
Yes in theory but the gain is not practical. You’re better off swapping batteries than trying to jury rig something. 

Let’s do the calculations. 

You note that the AC charger puts out 36VDC at 2 amps. That’s 72 watts per hour. 

Even a modest eBike has an 8Ah battery, for a total of 36x8 or 288 watt-hours. 

At 72 watt-hours per hour charging it takes four hours to charge. So you’re trying to fill your “gas tank” with a very tiny spigot, so to speak. 

Your internal batteries BMS charging  may stop taking a charge well below the output of your external pack. My 36v bike for example wants to see 42v on its input. So you’d need a boost circuit. 

Add to this charging inefficiencies which are usually on the order of 85-90% with each step and it’s not worth it. 

So just get another spare pack and exchange when your empty. Or rebuild your current pack so it’s larger but scale the BMS accordingly.