Q:
Is it possible to stack speed boost with...
A:
No. Movement speed bonuses only uses the highest value active. In your above example the Signet of Air at 25% would take precedence. The only benefit having Rune of the Traveler would provide is if you use the active ability of Signet of Air then you will be moving 5% faster instead of at the default rate.
Is it possible to stack passive speed boost to 30% with Signet of Air and Superior runes of the Traveler?
no, stacks in duration not intensity