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, input the measured concentrations to Lexogen's Library Quantification Calculation File (found here) 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 Kit for Illumina (Cat. No. 020) includes a Reamplification Primer that can be used to add some PCR cycles for your undercycled libraries (QuantSeq 3' mRNA-Seq User Guide, Appendix D). Please note that currently only single indexed libraries can be reamplified using the PCR Add-on Kit (Cat. No. 020).
Dual-indexed libraries can only be reamplified using the Reamplification Add-on Kit for Illumina (Cat. No. 080.96), which is available upon request by contacting support@lexogen.com. This Reamplification Add-on Kit can also be used to reamplify single-indexed libraries.