In many cases, undercycled libraries will still contain sufficient amounts of cDNA for pooling and sequencing and will not need to be reamplified.
To check whether your library amounts are sufficient for sequencing, you can use Lexogen's Library Quantification Calculation File (found here).
First, input the measured concentrations and adjust the amount of each library to be pooled (minimum = 10 fmol). So long as the volume indicated for pooling is below the volume of cDNA available, the library can be sequenced.
Should amounts of undercycled libraries be insufficient for sequencing, the PCR Add-on and Reamplification Kit V2 (Cat. No. 208) includes a Reamplification Primer that can be used to add some PCR cycles for your undercycled libraries (PCR Add-on and Reamplification Kit V2 User Guide, Section 6.1).