You can add collagen. The concentration we use is 1 mg/mL collagen in 2% alginate  for encapsulating hepatocytes. We also use tissue derived ECM. Here is the ref:

Journal of Biomedical Nanotechnology 08/2015; 11(8):1451-1460. DOI: 10.1166/jbn.2015.2085

Thanks for your answer. Did you see any changes in morphology of the cells with and without collagen?

try also cross linking with calcium ions, there are numerous papers on that.

well I crosslink alginate with cacl2 solution , is it what you mean?

I guess I am a little late to answer this but anyways maybe someone else can use it.

Alginate has carboxyl groups on its chain, so you can use the carbodiimide chemistry to crosslink alginate with the amine groups in the collagen or gelatin. There are lots of papers on this topic from the D. J. Mooney's group from Harvard.