Can I connect USB-C display's USB hub to a PC via USB 3.0 to female USB-C adapter?
The manual of the DELL S2718D says on p. 9
1x USB Type C (Alternate mode with DP1.2, USB 3.1 upstream port, Power Delivery PD up to 45W)
So it's not only a USB-C type port, it also needs alternate mode, so the USB host adapter in the computer you plug it into must be capable of this mode (and not all host adapters are).
Now it might be possible that the USB controller in the monitor is really really smart, and also supports an USB 2.0 connection, without power delivery or display port signals, where it just acts as a simple USB hub. In that case, an USB 2.0 to USB-C adapter will work.
But there's nothing in the manual that indicates it will do that, so the only way to find out is to either ask Dell, or try it out.
Edit
The manual of the LG 27UD88 says on p. 17:
The USB port on the product functions as a USB hub. Please connect the USB C-C or USB C-A cable to the PC.
So yes, for data only, it can use USB 2.0. But to display an image, you'll also need a PC capable of USB C alternate mode (even though this isn't explicitely mentioned in the LG manual, but that's the only way DisplayPort can be transported via USB C).
This is a little bit late (this post is Oct 2018) but perhaps it will be useful for others who might be researching this question.
I own a LG 38WK95C which has a USB-C upstream and 2x USB 3.0 downstream behind the monitor. There is also a included USB-A and USB-C adapter.
Two scenarios:
I plug keyboard and mouse into the USB 3.0 behind the monitor. I plug the USB-C cable into USB-C port in the MacBook. It charges the MacBook, it display the MacBook screen as a secondary display (via the USB-C) AND the keyboard and mouse work in the MacBook. It also carry sound, so the monitor speaker will play sound from the MacBook. I believe the USB "hub" is probably running in Alternate Mode which allows display, sound and data through the hub.
I unplug the USB-C from the MacBook and then plug into the C-A adapter, and then plug this cable+adapter into the PC USB port. The monitor display have to be powered by a separate cable connecting the monitor's DisplayPort port to the PC nvidia card (DP power the display and sound). The USB-C cable then enable the USB-C port behind the monitor to act as a upstream port, powering the hub and thus allowing me to use the same mouse and keyboard in Windows.
So, this hub works for both PC and Mac. The only inconvenience is that you have to physically unplug one cable from the PC to the Mac. But at least, the solution works and you only have one keyboard and mouse set connecting to the monitor.