Can I expand storage on iPad Pro 11 M1?

Sorry, but the built in internal storage size in your iPad Pro can't be changed.

However, you could use external storage devices:

Connect external storage devices to iPad - Apple Support

Sorry, no, not possible.

When purchasing an iPad, it is important to purchase as much internal storage as your budget might reasonably allow - as internal storage (and other hardware) cannot be changed, upgraded or extended. The available internal storage is fixed for the entire life of the device. If you run out of internal storage for your documents, photos and other data, you will forever struggle and become a source of frustration. It is far better to have more storage than you need, than to need more storage than you have.

A good rule of thumb is to quantify how much data storage you will need - add a healthy margin - then double it. If you are unable to quantify your data storage needs, it becomes more difficult. Whilst I and others cannot make definitive recommendations for your personal needs, objectively I would council against purchase of any iPad with any less than 256GB storage, 512GB and higher having a greater margin for growth. The 1TB and 2TB models of iPad Pro are the only models to offer these options at purchase, also benefiting from internal RAM being extended to 16GB; the iPad Pro M1 otherwise has 8GB RAM.

Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded, necessitating replacement of the device; by contrast, we never see complaint of having too much available storage. It is relatively easy to fill limited internal storage space; perhaps consider higher tiers of storage as providing room for growth as your needs develop or mature - potentially extending the useful usable life of the device. 

Also consider the functional differences between WiFi Only and WiFi+Cellular models of iPad. Aside from the obvious Cellular functionality, only WiFi+Cellular models include GNSS/GPS location capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. Purchase of a WiFi+Cellular model provides access to precise location services, but does not commit you to using Cellular connections. WiFi Only models of iPad have no Cellular or GNSS/GPS functions at all.

While your internal storage cannot be upgraded, you can make use of external USB storage - such as USB ThumbDrives and USB SSD devices - this being outlined with the often overlooked iPad User Guide. Here is a direct link to the relevant section of the Guide:

Connect external storage devices to iPad - Apple Support

You can also make use of Cloud Storage services - such as Apple iCloud, Google Drive, Microsoft One Drive etc. Each offer between 5-15GB free of charge, which can be extended for a small paid subscription - higher tiers obviously being more expensive.