I'm going to assume you mean CD34+ hematopoietic stem cells.
Although it is possible to expand hematopoietic progenitors in culture, to do this without any differentiation is not possible. Various hematopoietic growth factors will allow you to support the growth of stem cells into progenitors (KL, FL, TPO, GM-CSF, IL-3, IL-6, G-CSF). Other growth factors can help support various lineages of cells if that is what is desired. Generally, adding more cytokines supports more growth and more differentiation. There are small molecule agents such as the Aryl hydrocarbon receptor antagonist (e.g., StemRegenin 1) that can help minimize differentiation of stem cells. Bone marrow stroma (mesenchymal cells) or endothelial cells have also been shown to help support the maintenance of stem cells. Combinations of all these elements are also possible.