Can the transformer from an SMPS be used for step-up?
A transformer will work in either direction.  In each case, the open circuit voltage out the secondary will the that put into the primary times the number of secondary turns divided by the number of primary turns.  The only difference is the two windings flip primary and secondary roles when you use the transformer in opposite directions.

You need to look carefully at the impedance the transformer primary presents to the driving circuit, regardles of which direction the transformer is used in.  Without load on the secondary, the primary will look just like a inductor.  As the secondary is loaded, the primary will look lower impedance.  For a ideal transformer, the impedance on the secondary will be reflected on the primary divided by the square of the turns ratio.  For example, if the primary has 100 turns and the secondary 200, the turns ratio is 2:1.  The open circuit voltage will be multiplied by the turns ratio (it will be 2x of the primary at the secondary), and the impedance tied to the secondary will appear as 1/4 that on the primary.

As long as your source can drive whatever winding you choose as the primary and it does not overload the transformer, it will basically work.  Of course there will be losses, but getting into that gets a lot more complicated.