Actually , crossing is not always the best way to upgrade local chicken on spite of it is the fastest way. While doing crossing you may loose some of unique genes within your population. By each generation of crossing you decrease the percentage of local chicken genome within the offspring. So , by the fourth generation of crossing you will have chicken possess more genes from commercial lines than from those of local chicken. A detailed study and characterization is needed before any breeding program decision. good luck

The improvement of the local chicken passes by the knowledge of the characteristics of the latter. Indeed, it is necessary as a preliminary to estimate the parameters of production and reproduction of the local chicken, then to identify the race (genetic type) which has the best characters and to proceed to the crossing between the local chicken and the improved type. It's preferable to cross a local female in a male improved to preserve at the most the maternal characters of the local chicken

It is impossible to upgrade local chicken in a short span of time. For cross breeding you have to select a chicken male of high vigor and a female from local chicken, nurse their filial up to regular at least up to five generations to stabilize genome vigor.  It is true that few of the important genes may not be transferred  within population of your choice. From generations you should select two contrast characters hybridize them again and again. . By each generation of crossing you decrease the percentage of local chicken genome within the offspring. There is a possibility that after four and five generation of crossing, chicken may inherit more genes from commercial lines than from those of local chicken. 

what is your objectif? Many reasons make local chickens very interesting among which their rusticity and their ability to be integrated in local sustainable development. those advantages are related to their genes. I think that after a good characterization of local chicken, a selection could be very interesting to improve productivity depending of the objective of production. As said by previous, each generation of crossing will decrease the percentage of local chicken genome within the offspring and will lead to the situation of commercial chicken.

Ahmed and Cyrille's answers contain an important point. Local breeds or ecotypes will have evolved to suit the region they are grown in. Introducing 'exotic' birds or cross breeding may not be the best answer. This is something we are engaged in in East Africa and our preliminary data suggests that local animals have evolved to become resistant to the endemic diseases of the area and have characteristics that are advantageous to being produced in the area. If you have a facility with high biosecurity then commercial breeds may do fine, but local breeds are probably better in backyard-type production.