Question:  My City is replacing a fiber optic cable and a couple other cables, and because one cable is not going to be shipped to the site for some time, we are looking for options to pull most of the cables now and one later.  

Answer:  Pulling any cable into a conduit that already has cables can result in damage to the existing cables.  The probability of damage depends on the length of the conduit run, percent fill, number of existing cables and perhaps most importantly, the stiffness of the existing and new cables.  If the new cable can be hand-pulled and if the installers are careful, the risk of damage should be minimal.  

If any of the factors cited above indicate risk, innerduct can help.  Traditional innerduct is used to split one big conduit into multiple conduits.  A ribbed innerduct is shown below.  This product essentially creates multiple conduits instead of one big one.  

Another product called MaxCell is available for conduits where there is no room for traditional innerduct.  Maxcell can be pulled in the conduit over the existing cables, then one or more new cables can be added in the sleeves of the Maxcell.  Alternatively, the initial cables can be installed in one or more of the Maxcell sleeves, leaving one empty for the future cable.