Is it possible to Strengthen I-Joist?
Page 9 of the specification guide you linked describes Allowable Holes.  If you comply with that specification then you are not considered to have weakened the joist.  If you violate that specification then you would need new sealed engineering prints to stay compliant.

If you just want to increase rigidity or load-bearing capacity there are a number of common practices:


"Sistering" joists: I.e., running an identical joist alongside an existing joist.
Reducing the unsupported span of joists.  E.g., most simply, by adding a load-bearing post under a joist span.
Upgrading joists.  E.g., replacing existing members with joists having higher specified capacities.  Or adding supported joists under the existing ones.


You would need an engineer to quantify the effects of any of these reinforcements.  Some of these are very implementation-specific.  For example, depending on the construction, sistering might provide much more support when the sistered joists are cross-bolted in a particular way.