Midi learn for iOS Pianoteq 8

Click on "Options" near the upper right corner

Click on "MIDI" on the left

Adjust the volume slider on your keyboard.  If you see MIDI messages appearing in the top part of the window then your keyboard is sending MIDI messages for volume.  They would almost certainly be Control Change (CC) messages.  If not, then your keyboard doesn't send MIDI messages for volume and you probably can't do what you want except by using some other control on your keyboard other than the volume slider.  My keyboard, for example, doesn't send any MIDI messages when adjusting its volume knob.

If your volume slider does send MIDI messages, then in the bottom part of the window you can select the type of message in the left hand column, and in the middle column you can select "Set Parameter -> Volume" to create a mapping from the message to the volume setting.

If you click on the word VELOCITY above the graphical velocity curve on your PC there's an option to copy the curve.  That will convert it into a text form that looks something like Velocity = [0, 16, 34, 64, 93, 94, 119, 127; 0, 0, 24, 59, 97, 97, 127, 127].  You then need to get that text over to your iPad somehow.  You could paste it into an email or a message on a forum or social media to yourself, for example. In the worst case you could just type it into the Notes app on the iPad.  Then you would highlight the text on the iPad, copy it, and finally go over to Pianoteq, tap on VELOCITY, and select Paste.

One of the options in the menu that pops up when you click on VELOCITY is "Enable global velocity curves."