The Midrash says that Abraham and Sarah made many converts, presumably with God's blessing:
Abraham made converts, for it is written, "And Abraham took Sarai his wife... and all the souls that they had made in Haran." [Gen. 12:5] R. Eleazar observed in the name of R. Yosei ben Zimra: If all the nations assembled to create one insect, they could not give it life, yet you say, "And all the souls that they had made in Haran!" It refers to the converts. Then let [the Torah] say, "That they had converted". Why "That they had made?" That is to teach you that if one brings a convert near [God], it is as though he had created him. Now why does it not say "That he had made" instead of "That they had made"? Said R. Hunia: Abraham converted the men and Sarah the women. [Genesis Rabbah 84:4]
But elsewhere the Midrash says that God did not want converts to join the Israelites in the Exodus:
[The Torah says: The Lord spoke to Moses, “Hurry down, for your people, whom you brought out of the land of Egypt, have corrupted.” [Ex. 32:7]] [God said:] It does not say the people but your people. Moses exclaimed: Lord of the Universe! On what basis are they my people? God replied: They are indeed your people, for when they were yet still in Egypt, I told you that I will bring forth My legions, My people, the children of Israel [Ex. 7:4]. Did I not tell you not to allow a mixed multitude to be mingled with them? But you, being humble and righteous, responded to Me, “Those who repent must always be accepted.” Now, I knew what they would do [to Israel] in the future, but did I tell you that [to change your mind]? No, I fulfilled your wish, and the result was that it was just these people who made the [Golden] Calf, for they had been idol worshipers [before]. [Exodus Rabbah 42:6]
So which is it? Did God want converts or not before the Jews became a nation? Was God telling Moses, in effect, "Before you can teach others, you have to learn yourselves. Wait until I give you the Torah, wait until you live with it for a while and make it part of your lives, wait until you can stand firmly on your own two feet, and then you can bring strangers into the fold. Right now, the strangers are more likely to influence you than you are to influence them."